Head centering jig for femoral resurfacing

ABSTRACT

There is described a jig for identifying a point on the femoral head which is in alignment with a central axis of the femoral neck. The jig comprises a first member which defines a plane and has means for at least partially receiving the femoral neck. The jig also comprises a guide member which is mounted apart from the first member and which defines an axis at right angles to the plane defined by the first member. The axis of the guide member intersects the plane at a point which is a predetermined distance away from the means receiving the femoral neck. In addition, the jig comprises an elongate alignment means which is mounted with respect to the first member and which is spaced apart from, but extends parallel to, the axis of the guide member. There is also described a kit for use in the resurfacing of the femoral head. The kit comprises at least one jig as previously described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a divisional of U.S. patent application Ser.No. 10/44,152, filed on May 12, 2004, which claims priority to UnitedKingdom Patent Application No. 0310869.3, which was filed on May 12,2003.

FIELD OF THE INVENTION

The present invention relates to a head centering jig for use in thepreparation of the femur in hip resurfacing procedures and to a kitcomprising such a jig.

BACKGROUND OF THE INVENTION

Conventional Total Hip Replacement (THR) is a very successful procedurefor the treatment of arthritis of the hip, a condition which causesconsiderable pain and loss of movement. As is well known, the hip is aball and socket joint which allows the upper leg to move from side toside, back to front, and to rotate. The joint is made up of the head ofthe femur (the ball) which fits into the acetabulum (the socket). In ahealthy hip, both the head of the femur and the acetabulum are coveredwith cartilage which provides a smooth surface allowing the joint tomove freely.

The earliest Total Hip Replacement (THR) procedure, in 1938, involved animplant in which both surfaces were made from metal and was known as ametal-on-metal bearing. During the 1950's and early 1960's, a number ofsurgeons developed hip implants using this type of bearing, althoughmany of these designs had a tendency to work loose early on as thetechniques used to anchor them to the bone were not very successful.However, the implants that did not loosen early on have generally lastedwell.

In the early 1960's, a British Surgeon, Sir John Charnley, developed anew type of Total Hip Replacement which is still in use today. Thisprocedure, which is illustrated in FIG. 1, involves cutting the wornhead off the femur and replacing it with a metal ball 10 and stem 12 inthe shaft of the femur 14 and a plastic cup 16 in the pelvis 18. Bothelements are typically anchored to the bone by “bone cement”. This hasbecome a very common surgical procedure with some 45,000 hipreplacements being carried out in the UK every year.

The plastic used to form the cup 16 is inert and so is well tolerated bythe body. Nevertheless, as the metal ball 10 rubs against the plasticcup 16, tiny particles of the plastic are worn away. This plastic debriscauses an irritation. Furthermore, as the particles get between the boneand the artificial joint, this irritation causes surrounding bone to beabsorbed by the body, leading to a loosening of the artificial joint. Inolder people, with a lower activity level, this may not happen fortwenty or more years, but in younger, more active patients, this mayhappen much sooner.

To overcome these problems in younger, more active patients requiringhip replacement, a different type of implant was needed. In 1991 aprocedure for metal-on-metal (MoM) resurfacing of the hip was proposed.This has two major differences from a conventional THR. The first isthat both components are made from metal, typically Cobalt Chrome. Byeliminating the plastic cup of a conventional THR, and making both partsof the bearing surface of metal, the resurfaced hip is expected to lastmuch longer and therefore to be more suitable for higher demandingpatients. The second difference is that the procedure is very boneconserving, since the head of the femur is simply reshaped and“resurfaced”, rather than removed. Accordingly, should the device needreplacing at some time in the future, this may provide better optionsfor the surgeon at that time as a conventional THR can then be used.

A typical Resurfacing Hip is shown in FIG. 2 to comprise a femoral headcomponent 20 and an acetabular cup 22. During the pre-operative planningstage, an X-ray of the hip is templated to assess the probable sizes forboth the femoral head component 20 and acetabular cup 22. Alignment ofthe femoral head component 20 is also determined pre-operatively and isan important part of the templating procedure. The femoral headcomponent 20 should be positioned in neutral or slight valgus alignment.Varus positioning should be avoided. Once the template has beensatisfactorily positioned with respect to the X-ray, the distance ismeasured from the tip of the greater trochanter 24 to the point wherethe axis 26 of the femoral neck 32 crosses the lateral cortex of thefemoral shaft 14. This pre-operative measurement is recorded as itindicates the position for subsequent insertion of a lateral positioningpin 28 of a head centering jig 30 (see FIG. 3 and related descriptionbelow).

Initial preparation of the femur may be carried out to one size largerthan that templated in order to ensure that there is sufficientclearance around the femoral neck 32 for the definitive implant. Furtherpreparation to the templated size may then be carried out if it isevident that there is sufficient clearance around the neck 32 and thatthe integrity of the neck will not be compromised. Sufficient clearancemay occasionally still remain to allow a smaller size to be used thanwas suggested by templating. In this case, further preparation to thesmaller size may be carried out, ensuring the use of the mostappropriate size of femoral components and minimizing the amount of boneremoved from the acetabulum 34.

During hip resurfacing procedures it is common to prepare the femurfirst as this will debulk the femoral head 36 and facilitate access tothe acetabulum 34. As part of this process the mid-lateral cortex of thefemur is exposed and the position of the lateral positioning pin 28 isdetermined based on the pre-operative measurement taken from the X-ray.The lateral positioning pin 28 is then drilled into the mid-lateralcortex of the femur, initially using a lateral approach, but angling thepin towards the femoral head 36 once the outer cortex has beenpenetrated, as shown in FIG. 3.

Before proceeding with head centering, the size of the femoral headcomponent 20 can be confirmed by placing an appropriate head templatearound the femoral neck 32. What is important to assess is that theappropriate clearance is available and that the femoral neck 32 will notbe notched during preparation of the femur since this will result in apotential post-operative weakening of the hip.

In the past, once the size of the femoral head component 20 had beenconfirmed, a jig 30, such as that shown in FIG. 4, would be used tolocate the centre of the femoral head 36. Since the femoral head 36 doesnot define a uniform sphere, what is important is to identify that pointon the femoral head which coincides with the central axis 26 of thefemoral neck 32.

As shown, the head centering jig 30 of the prior art comprises a hollowguide tube 38 having a proximal end 40 and a distal end 42. The guidetube 38 is supported by an arm 44 which locates at one end over thelateral positioning pin 28 and is provided at the other with a pawl 46for selective engagement with a rack 48 provided on an outer surface ofthe guide tube. Thus, by the selective engagement and disengagement ofthe ratchet mechanism defined by the pawl 46 and rack 48 the guide tube38 may be progressively advanced with respect to the arm 44. A lockingscrew 50 is provided to retain the guide tube 38 in fixed relation tothe arm 44 once the desired relative position has been established.

A sleeve 52 is rotatably mounted to the guide tube 38 at a positionintermediate the distal end 42 and the rack 48. As well as rotating, thesleeve 52 is also able to slide along the guide tube 38, therebyenabling the sleeve to be positioned at a range of distances from thedistal end 42. A further locking screw 54 is provided to enable thesleeve 52 to be clamped longitudinally with respect to the guide tube 38while still permitting a portion of the sleeve to rotate. To thisrotating portion there is attached a projection 56 which extends in adirection perpendicular to a longitudinal axis of the guide tube 38. Astylus 58 is slidably mounted on the projection 56 to which it may beclamped in a selected one of a plurality of predetermined positions bymeans of a third locking screw 60. These predetermined positionscorrespond to different sizes of femoral head component 20 and aremarked on the projection 56 as a series of graduations.

In order to identify the centre of the femoral head 36 the stylus 58 isset to the confirmed size of the femoral head component 20 and the thirdlocking screw 60 is tightened to clamp the stylus with respect to theprojection 56. The aim 44 of the head centering jig 30 is then locatedover the lateral positioning pin 28 and the guide tube 38 advancedtowards the femoral head 36 in a controlled manner by means of theratchet mechanism defined by the pawl 46 and rack 48. Centralization ofthe guide tube 38 is achieved by rotating the stylus 58 around thefemoral neck 32. It is critical that a tip of the stylus 58 rotatesfreely around the femoral neck 32 at the head/neck junction withoutimpingement. In this way it is possible to avoid subsequent notching ofthe femoral neck 32. In order to assess more easily whether the guidetube 38 is accurately centered with respect to the femoral head 36, thesleeve 52 may be slid along the guide tube to a point where the tip ofthe stylus 58 coincides with the junction of the femoral neck 32 andfemoral head 36. The further locking screw 54 may then be tightened toretain the sleeve 52 in this longitudinal position with respect to theguide tube 38.

Once the guide tube 38 has been properly centered it can be locked intoposition by gently impacting into the femoral head 36 a plurality ofcircumferential teeth (not shown) provided on the distal end 42. A longpin 62 is then drilled through the guide tube 38 into the femoral head36 before the head centering jig 30 is then disassembled and removed. Ifrequired, once the guide tube 38 has been disassembled from the arm 44,the guide tube and stylus 58 can be positioned over the long pin 62 tocarry out a final check that the long pin is correctly positioned inrelation to the femoral neck 32.

Thereafter, as shown in FIG. 5, a cannulated drill 64 is advanced overthe long pin 62 as far as an appropriate line for the size of thefemoral head component 20 being used. Both the cannulated drill 64 andthe long pin 62 are then removed and a guide rod 66 inserted in theirplace.

As shown in FIG. 6, an appropriate size of head cutter 68 is advancedover the guide rod 66 as far as the junction between the femoral head 36and femoral neck 32, ensuring that the femoral neck is not notched.During this process, the head template can be positioned around thefemoral neck 32 to protect the neck and trochanter 24, while swabs canbe used to prevent bone debris entering the soft tissue. If necessary, alarger size of head cutter 68 may be used for initial preparation of thefemoral head 36 prior to final preparation with the definitive size inorder to debulk the head.

Having removed the head cutter 68, the guide rod 66 is also then removedand a top head guide 70 placed over the prepared head surface andadvanced to the head/neck junction. As shown in FIG. 7, a locking screw72 is provided to hold the top head guide 70 in place while the top ofthe femoral head is resected. If necessary, a short pin (not shown) canbe impacted through a hole in the top head guide 70 if additionalsecurity is required during the head resection. Following resection theguide rod 66 is reinserted and a top head cutter 74 advanced along theguide rod to create a flat surface perpendicular to the neck axis 26. Asa result, the femoral head 36 acquires the shape shown in FIG. 8. Anappropriate size of head chamfer cutter 76 is then used to create abevel as shown in FIG. 9. Having thus shaped the femoral head 36, acorresponding size of head template 78 is used to check the preparedshape as shown in FIG. 10 and to make a mark on the head/neck junctionto indicate how far the femoral head component 20 should be advanced iffully seated. Cement keyholes can be drilled into the femoral head 36 atthis stage if required.

Having prepared the acetabulum 34 and fitted the acetabular cup 22, thefemoral head component 20 is similarly implanted to the prepared femoralhead 36. If a cemented head is used, low viscosity bone cement is pouredinto the femoral head component 20 up to a line at the top of a chamferon the inside of the implant. The definitive component 20 is then placedonto the femoral head 36 and impacted into position using a headimpactor 80 as shown in FIG. 11. Any excess bone cement may then beremoved. Low viscosity cement is used in preference to high viscositycement since high viscosity cement may prevent full seating of thefemoral head component 20. However, cementless head components are alsoavailable and these are simply impacted into position using the headimpactor 80.

With both components now fitted, the lateral positioning pin 28 isremoved and the hip reduced while at the same time avoiding scratchingthe femoral head component 20 against the rim of the acetabular cup 22.A full check is then made to ensure that there is no impingement andthat the range of movement and stability are satisfactory.

Thus, it can be seen that accurately determining the centre of thefemoral head 36 is a critical step in a hip resurfacing procedure sinceit determines not only the position of the long pin 62 and the guide rod66 but also the relative positions of all the cutters and guides thatare subsequently used to shape the femur. There are, however, a numberof problems with the prior art head centering jig shown in FIG. 3.

Firstly, the prior art jig 30 requires the use of a lateral positioningpin 28 which must be correctly positioned with respect to the femoralneck 32. This not only requires pre-operative planning but also theintra-operative exposure of the mid-lateral cortex of the femur.Furthermore, having been inserted, it becomes necessary to remember toremove the lateral positioning pin 28 from the femoral cortex before thewound is closed. Although this might seem obvious, experience has taughtthat it is a sensible precaution to attach a chain 82 or other reminderto the lateral positioning pin 28 to ensure that its removal is notoverlooked.

Another problem with the prior art head centering jig 30 is that it isbulky since the arm 44 must extend from the lateral positioning pin 28at the mid-lateral femoral cortex to the femoral head 36 while stillallowing the stylus 58 to rotate about the guide tube 38.

The prior art head centering jig 30 also requires the use of two hands,one to hold the guide tube 38 and the other to rotate the stylus 58.

It would therefore be advantageous to provide an improved head centeringjig which was less bulky and could be operated with only one hand. Itwould also be advantageous if the use of the head centering jig nolonger necessitated exposure of the mid-lateral cortex of the femur.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided ajig for identifying a point on the femoral head in alignment with acentral axis of the femoral neck, the jig comprising a first memberdefining a plane and having means for at least partially receiving thefemoral neck; a guide member mounted in spaced relation with respect tothe first member and defining an axis at right-angles to the planedefined by the first member, the axis intersecting the plane at a pointa predetermined distance from said means for at least partiallyreceiving the femoral neck; and elongate alignment means mounted withrespect to the first member, the alignment means being spaced from andextending parallel to the axis defined by the guide member.

Advantageously, the guide member may comprise a hollow tube and the axisdefined by the guide member may comprise a central axis of the tube.This provides for accurate positioning of the drill required to drillthe long pin into the femoral head. Alternatively, the guide member maycomprise a channel member. This provides the advantage of not having todisassemble the jig prior to its removal once the long pin has beendrilled into the femoral head.

Advantageously the guide member may be mounted with respect to the firstmember so as to selectively detachable therefrom. This facilitates thedisassembly of the jig. Preferably the guide member may be separablefrom the first member in a direction parallel to the axis defined by theguide member. This facilitates the removal of the guide member over thelong pin once the long pin has been drilled into the femoral head.

Advantageously the first member may comprise a handle. This facilitatesthe ease with which the jig may be manipulated by a surgeon. Preferablythe means for at least partially receiving the femoral neck may beadapted to so receive the femoral neck when the means is offered up tothe femoral neck in a first direction, the handle extending in adirection parallel to the first direction. This increases the ease withwhich the jig may be used. Preferably the elongate alignment means maybe spaced from the axis defined by the guide member in a seconddirection, the second direction being orthogonal to the direction inwhich the handle extends. Likewise, when the means for at leastpartially receiving the femoral neck is adapted to so receive thefemoral neck when the means is offered up to the femoral neck in a firstdirection, the elongate alignment means may be spaced from the axisdefined by the guide member in a second direction, the second directionbeing orthogonal to the first direction. This facilitates the alignmentof the guide member with the central axis of the femoral neck.

Advantageously the elongate alignment means may be adapted to be mountedwith respect to the first member in a selected one of two positions, thetwo positions being spaced on opposite sides of the axis defined by theguide member. This enables the jig to be adapted depending on whetherthe surgeon is either left handed or right handed or whether the femoralhead that is being reshaped is the patient's left or right. Preferablythe two positions may be mirror images of each other.

Advantageously the elongate alignment means may extend through the planedefined by the first member. This further facilitates the alignment ofthe guide member with the central axis of the femoral neck.

Advantageously the elongate alignment means may be adapted to be mountedwith respect to the first member in a selected one of a plurality ofpositions spaced at different distances from the axis defined by theguide member. This enables the jig to accommodate femoral heads ofdifferent dimensions and enables the elongate alignment means to bespaced at different distances from the femoral neck in accordance withthe preference of the surgeon.

Advantageously the elongate alignment means may comprise at least onerod. Where the elongate alignment means comprises first and second rodsmounted with respect to the first member and extending parallel to andspaced from the axis defined by the guide member, the first rod may bespaced from the axis in a direction orthogonal to that in which thesecond rod is spaced from the axis. This facilitates the alignment ofthe guide member with the central axis of the femoral neck by providinga means for obtaining simultaneous alignment in two orthogonal planes.Where the elongate alignment means comprises first and second rods andwhere the first member comprises a handle, one of the first and secondrods may extend along an axis that intersects the handle.

Advantageously, the first member may comprise means adapted tosubstantially encircle the femoral neck. This enables the first memberto be more accurately positioned with respect to the femoral neck.Preferably the means for substantially encircling the femoral neck maycomprise a first part hingedly mounted at a first end with respect to asecond part and moveable between an open position in which an oppositeend remote from the hinge is separated from the second part by adistance sufficient to receive the femoral neck and a closed position inwhich the parts define a wall adapted to substantially surround thefemoral neck. Preferably the wall defined by the first and second partsin the closed position subtends an angle of 270° or more. Preferably, inthe closed position, the arc subtended by that part of the wall definedby the first part is substantially the same as the arc subtended by thatpart of the wall defined by the second part.

Preferably, in the closed position, the first and second parts maydefine a circular aperture for the receipt of the femoral neck. Undersuch circumstances the axis defined by the guide member may intersectthe plane defined by the first member at the centre of the circularaperture defined by the first and second parts. This ensures that whenthe elongate alignment means is aligned with the femoral neck, the axisdefined by the guide member coincides with the central axis of thefemoral neck.

Advantageously means may be provided to releasably retain the first andsecond parts in the closed position. This provides the advantage ofpreventing the first member from becoming accidentally dislodged fromthe femoral neck. Preferably the first and second parts are eachprovided with a respective aperture, the aperture of the first partbeing adapted to be aligned with the aperture of the second part whenthe first and second parts are in the closed position, the mutuallyaligned apertures being adapted to receive the means for releasablyretaining the first and second parts in the closed position. Preferablythe guide member comprises the means adapted to be received within themutually aligned apertures to releasably retain the first and secondparts in the closed position.

According to a second aspect of the present invention there is provideda kit for use in the resurfacing of the femoral head, the kit comprisingat least one jig as previously described.

Advantageously, the kit may comprise a plurality of jigs, each jig beingas previously described and differing from each other in terms of thedimensions of the means for at least partially receiving the femoralneck and/or the predetermined distance from said means at which the axisdefined by the guide member intersects the plane defined by the firstmember. This provides the advantage of enabling the kit to accommodatethe range of femur sizes present in the population.

According to a third aspect of the present invention there is provided amethod of identifying a point on the femoral head in alignment with acentral axis of the femoral neck, the method comprising the steps of:providing a jig comprising a first member defining a plane and havingmeans for at least partially receiving the femoral neck, a guide membermounted in spaced relation with respect to the first member and definingan axis at right-angles to the plane defined by the first member, theaxis intersecting the plane at a point a predetermined distance fromsaid means for at least partially receiving the femoral neck, andelongate alignment means mounted with respect to the first member, thealignment means being spaced from and extending parallel to the axisdefined by the guide member, exposing the femoral head and neck;offering up the first member to the femoral neck so that the femoralneck is at least partially received by said means for at least partiallyreceiving the femoral neck; aligning the elongate alignment means with acentral axis of the femoral neck; and identifying a point at which theaxis defined by the guide member intersects the femoral head as thepoint on the femoral head in alignment with the central axis of thefemoral neck.

Preferably the mid-lateral cortex of the femur is not exposed.

Advantageously the first member is offered up to the femoral neck andthe elongate alignment means is aligned using only one hand. Preferablythe first member is offered up to the femoral neck in a lateralapproach.

Preferably the step of offering up the first member to the femoral neckcomprises substantially encircling the femoral neck.

Advantageously the step of aligning the elongate alignment meanscomprises aligning the elongate alignment means with a central axis ofthe femoral neck when viewed from two orthogonal directions.

Preferably the method further comprises the step of drilling along theaxis defined by the guide member and into the femoral head. Morepreferably the method further comprises the step of disassembling thejig prior to removal from the femoral neck.

Advantageously the jig is provided as part of a kit comprising aplurality of jigs, each jig differing dimensionally from each other jig,the method further comprising the step of selecting the appropriate jigfor the size of femoral head to be surveyed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary frontal view of a hip replacement according to aprocedure of the prior art.

FIG. 2 is a fragmentary frontal view of a hip replacement according to aprocedure of the prior art.

FIGS. 3-11 are fragmentary frontal views of hip replacement proceduresand tools of the prior art.

FIG. 12 is a perspective view of a head centering jig in accordance witha first embodiment of the present invention;

FIG. 13 is a plan view of a locating tool used in the first embodimentof the present invention;

FIG. 14 is a cross-sectional view of a retaining member used in a firstembodiment of the present invention;

FIG. 15 is a plan view of a collar used in a first embodiment of thepresent invention;

FIG. 16 is a side view of an alignment rod used in a first embodiment ofthe present invention;

FIG. 17 is a perspective view of a head centering jig in accordance witha second embodiment of the present invention;

FIG. 18 is a plan view, partially in cross-section, of a locating toolused in a second embodiment of the present invention;

FIG. 19 is a plan view of a locating tool comprising two arms used in athird embodiment of the present invention in a closed position;

FIG. 20 is an exploded perspective view of the locating tool of FIG. 19;and

FIG. 21 is a perspective view of the locating tool of FIGS. 19 and 20 inan open position.

DETAILED DESCRIPTION OF EMBODIMENTS

Referring to FIGS. 12 to 16 there is shown a head centering jig 100comprising a substantially planar locating tool 102. In the embodiment,shown the locating tool 102 has a spanner-like form and is symmetricalabout a longitudinal axis 103. The locating tool 102 comprises anelongate handle 104 having a rounded proximal end 106. At a distal end,the elongate handle 104 merges with a planar head portion 108. The headportion 108 is recessed at an edge opposite where the head portionmerges with the handle 104 so as to define a mouth 112 having a pair ofmutually spaced jaws 110 which extend away from the handle on eitherside of the longitudinal axis 103. In order to minimize the traumacaused in using the head centering jig 100, the head portion 108 isprovided with an external profile devoid of sharp angles. Indeed, insome preferred embodiments, the external profile of the head portion 108may be smoothly curved. By contrast, the mouth 112 is defined by a pairof confronting surfaces 105 and 107 which extend parallel to and areequally spaced from the longitudinal axis 103 and an orthogonal surface109 which extends perpendicularly to the longitudinal axis and which, insome embodiments, may serve to join the confronting surfaces 105 and107. In other embodiments, such as that illustrated, the orthogonalsurface 109 is joined to the confronting surfaces 105 and 107 viarespective angled surfaces 111 and 113 so that all of the surfacesdefining the mouth 112 subtend an oblique angle with the surface orsurfaces adjacent to them. In this way the mouth 112 has an internalprofile which, in plan view, approximates to a U-shape. In someembodiments this internal profile may be made smoother still such thatthe confronting surfaces 105 and 107 are joined by an arcuate orsemicircular surface (not shown).

A retaining member 114 is releasably mounted to the locating tool 102and is carried on an upper surface of the handle 104 adjacent the distalend where the handle merges with the head portion 108. To this end, thehandle 104 is provided with three apertures 115, 117 and 119 which arespaced in the longitudinal direction along the longitudinal axis 103. Inuse the central of these three apertures 117 communicates with athreaded bore 121 provided in an undersurface of the retaining member114. As a result, a complimentary threaded locking screw 116 may passthrough the central aperture 117 to engage the threads of the blind bore121 and in so doing releasably secure the retaining member 114 withrespect to the locating tool 102. In order to facilitate the alignmentof the retaining member 114 with respect to the locking tool 102, thesame undersurface having the blind bore 121 is also provided with twoprojecting studs 118 and 120. When the retaining member 114 is correctlyaligned with respect to the locking tool 102, these projecting studs 118and 120 are received within the outer pair of the apertures 115 and 119provided in the handle 104.

As can be seen, the retaining member 114 comprises an elongate foot 122which is releasably attached to the locating tool 102 by means of thelocking screw 116 and extends in a direction coincident with thelongitudinal axis 103, an upstanding member 124 which merges with thefoot 122 and projects upwardly from the handle 104 perpendicular to theplane defined by the locating tool 102, and an arm 126 which merges withthe upstanding member 124 at an end remote from the foot 122 and extendsin a plane parallel to and spaced from that defined by the locating tool102. The arm 126 extends in a direction parallel to the longitudinalaxis 103 and, at an end remote from the upstanding member 124,terminates in an arcuate surface. This enables the arm 126 to abut, andin some embodiments be welded, brazed or otherwise joined to thecylindrical outer surface of a hollow guide tube 128.

In those embodiments in which the guide tube 128 is not joined to thearm 126, the guide tube 128 may nonetheless be secured to the retainingmember 114 by means of a collar 130 which surrounds the guide tube 128and retains the guide tube in a friction fit and is itself attached tothe arm 126 by means of a locking screw 132. Thus, the collar 130, aswell as having a central aperture 131 for receipt of the guide tube 128,is provided with a first radial projection 134 having a through bore 133located at an end remote from the collar. When the guide tube 128 abutsthe arcuate end surface of the arm 126 this through bore 133communicates with a threaded bore 135 provided in the arm such that thecomplimentary threaded locking screw 132 may pass through the firstradial projection and into threaded engagement with the bore provided inthe arm 126.

With the guide tube 128 joined or secured in this position, a centralaxis 136 of the guide tube intersects the plane defined by the lockingtool 102 at a location along the longitudinal axis 103 and within themouth 112 a predetermined distance from the orthogonal surface 109. Inthose embodiments in which the internal profile of the mouth 112approximates to a U-shape, this predetermined distance may beapproximately equal to half the distance separating the confrontingsurfaces 105 and 107. Likewise, in those embodiments in which theconfronting surfaces 105 and 107 are joined by a semicircular surface,the central axis 136 of the guide tube 128 may intersect the planedefined by the locating tool 102 at the centre of the circle defined bythat semi-circular surface. In those embodiments in which theconfronting surfaces 105 and 107 are joined by an arcuate surface, thepoint of intersection may coincide with the centre of curvature of thearcuate surface.

In addition to the first radial projection 134, the collar 130 is alsoprovided with a second radial projection 138. This second radialprojection 138 is longer than the first and extends at right-angles toit. By definition therefore, the second radial projection 138 alsoextends at right-angles to the arm 126. A plurality of apertures 140 areprovided at intervals along the centre line of the second radialprojection and are sized so as to slidably receive one end 142 of afirst alignment rod 144. As can be seen, the alignment rod 144 is ofcircular cross-section and has a diameter that is stepped at a locationintermediate the ends of the rod such that, at one end 142, thealignment rod has a diameter capable of being received through aselected one of the apertures 140 and, at the other end 146, has adiameter which is not so capable. As a result, an annular shoulder 145is formed at the location where the diameter of the alignment rodincreases and it is this shoulder which abuts an upper surface of thesecond radial projection 138 and prevents the alignment rod 144 frompassing completely through the selected aperture 140.

Although not strictly necessary, a second alignment rod 148, of similarconstruction to the first, may be arranged to pass through a selectedone of a plurality of mutually aligned apertures 150 provided in thefoot 122 and arm 126 of the retaining member 114 and in the handle 104of the locating tool 102. As will be readily understood, both alignmentrods 144 and 148 extend parallel to the central axis 136 of the guidetube 128 and perpendicular to the plane defined by the locating tool102. Furthermore, in the same way that the first alignment rod 144 maybe spaced from the axis 136 at different distances depending on theaperture 140 through which the alignment rod is received, so too may thesecond alignment rod 148 by appropriate selection of the mutuallyaligned apertures 150.

One of the advantages of the head centering jig 100 shown in FIGS. 12 to16 is that, unlike the jig of the prior art, it does not require theprovision of a lateral positioning pin. As a result the mid-lateralcortex of the femur need not be exposed during the Hip Resurfacingprocedure and there is no risk of a positioning pin being left behindonce the wound is closed. Instead, the only parts of the femur that needbe exposed are the femoral head and femoral neck. Accordingly, once thishas been achieved, in order to identify the point on the femoral head inalignment with the central axis of the femoral neck the surgeon simplyselects a head centering jig appropriate to the patient. In order toaccommodate the range of femur sizes present in the population it willbe necessary to provide a range of jigs which differ in terms of thedimensions of the mouth 112. The length of the arm 126 would also varyfrom jig to jig in order to maintain the relationship between thecentral axis 136 of the guide tube 128 and its point of intersectionwith the plane defined by the locating tool 102. Nevertheless, thisrange of jigs may be made so that each jig 100 corresponds to arespective one of the standard sizes of femoral head component.

The head centering jig 100 is offered up to the femur using a lateralapproach. In so doing, by making use of the elongate handle 104 thesurgeon is able to grip the jig using only one hand. This representsanother significant advantage over the head centering jigs of the priorart. The locating tool 102 is positioned so that the femoral neck isreceived between the mutually spaced jaws 110 and abutting theorthogonal surface 109. With the femoral neck thus received within themouth 112, the plane defined by the locating tool 102 is tilted byraising and lowering the handle 104 until such time as the firstalignment rod 144 is aligned parallel to the central axis of the femoralneck when viewed anteriorly. Assuming that the appropriately sized jighas been selected and that the femoral neck is a close fit between thejaws 110 of the locating tool 102, the central axis 136 of the guidetube 128 will lie within a plane parallel to the coronal plane. By thenaligning the first alignment rod 144, the central axis 136 is broughtinto coincidence with the central axis of the femoral neck. Thus, usinghis free hand, the surgeon may then drill the long pin through the guidetube 128 and into the femoral head.

If additional alignment is required and if the head centering jig 100 isprovided with a second alignment rod 148 then, once the first alignmentrod 144 has been aligned with the central axis of the femoral neck whenviewed anteriorly, the second alignment rod 148 may be aligned with acentral axis of the femoral neck when viewed laterally. This alignmentof the second alignment rod 148 ensures that the central axis 136 of theguide tube 128 lies within a plane parallel to the coronal planenotwithstanding any small amount of play between the femoral neck andthe jaws 110 of the locating tool 102 and, if performed simultaneouslywith the similar alignment of the first alignment rod 144, ensures thatthe central axis 136 coincides with the central axis of the femoralneck. However, as stated previously, assuming that the appropriate jig100 has been selected and the jaws 110 are appropriately spaced, thissecondary alignment should not be necessary. Even if it does provenecessary, the surgeon may choose to align the upstanding member 124with the central axis of the femoral neck to the same effect rather thanemploy a second alignment rod 148.

Once the long pin has been drilled into the femoral head the headcentering jig 100 is disassembled and removed. In order to achieve this,the locking screw 116 is unscrewed from the blind bore 121 therebyenabling the retaining member 114 to be separated from the locating tool102. The retaining member 114, the guide tube 128 and the firstalignment rod 144 are then removed over the long pin in a directionparallel to the central axis 136. If a second alignment rod 148 isprovided then this too is removed from the locking tool 102 in adirection parallel to the central axis 136. By contrast, the lockingtool 102 is removed laterally in a direction perpendicular to the longpin. With the head centering jig 100 removed from the wound, theremainder of the Hip Resurfacing procedure may be performed in themanner previously described with a cannulated drill being advanced overthe long pin as the next step in the reshaping of the femoral head.

Although the head centering jig 100 has been described as comprising ahollow guide tube 128 defining a central axis 136, it will be apparentthat in certain embodiments the hollow guide tube may be replaced by achannel member of U-shaped or V-shaped cross-section sized to receive adrill for drilling the long pin into the femoral head and capable ofuniquely defining a drill axis perpendicular to the plane of thelocating tool 102. The advantage of such an embodiment is that thechannel member does not surround the drill axis so that once the longpin has been drilled into the femoral head, the channel member,retaining member and locating tool may be removed from the femur in asingle operation and without requiring the disassembly of the headcentering jig.

Another advantage of the present head centering jig 100 is that thefirst alignment rod 144 may be located with respect to the guide tube128 on either side of the retaining member 114. This may be usefuldepending on the handedness of the surgeon or whether the femoral headthat is being reshaped is the patient's left or right. Thus, forexample, by unscrewing locking screw 132 from threaded bore 135, thecollar 130 may be removed from the arm 126 and inverted before beingreassembled to the retaining member 114. In this way, the second radialprojection 138, instead of projecting to the right when viewed along theaim 126 in the direction of the guide tube 128, as shown in FIG. 12,projects to the left.

A head centering jig 200 in accordance with a second embodiment of thepresent invention is shown in FIGS. 17 and 18. Many of the features ofthe second embodiment, including the retaining member 114, the guidetube 128 and the two alignment rods 144 and 148 are similar to thosedescribed in relation to the first embodiment and so will not be furtherdescribed in relation to the second embodiment in which they are denotedby the same reference numerals. Nevertheless, the locating tool 202 ofthe second embodiment does differ from that of the first embodimentalthough, like the first embodiment, it comprises an elongate handle 204having a rounded proximal end 206.

Although the retaining member 114 may be secured to the handle 204 bymeans of a locating screw 116 in the same manner as has been describedwith reference to the first embodiment and, indeed, as illustrated inFIG. 17, the retaining member 114 may equally be attached to the handle204 by means of the fixation mechanism illustrated in FIG. 18. In thisarrangement the handle 204 is again provided with a pair of apertures115 and 119 for the receipt of the projecting studs 118 and 120 providedon an undersurface of the retaining member 114. However, whereas in thefirst embodiment the retaining member 114 is releasably secured to thelocating tool 102 by means of a threaded locking screw 116 passingthrough the handle 104 in a direction perpendicular to the plane definedby the locating tool 102, in the embodiment shown in FIG. 18 a lockingscrew 201 passes through a threaded aperture located within the plane ofthe handle 204 and extending along an axis 203 passing through thecenters of the apertures 115 and 119. Indeed, the threaded aperturecommunicates with aperture 119 and, as a result, the locking screw 201may be tightened so that an end of the locking screw 205 bears againstthe projecting stud 120 thereby releasably securing the retaining member114 with respect to the locating tool 202.

At a distal end, the handle 204 merges with one end of a first arcuatemember 208 which defines an arcuate surface 210 having a centre ofcurvature 212 which lies on the axis 203. A second arcuate member 216 ishingedly connected to the first arcuate member 208 at an end remote fromthe handle 204 and defines an arcuate surface 218 having the same centreand radius of curvature as the arcuate surface 210. Indeed, betweenthem, the two arcuate surfaces 210 and 218 subtend a combined arc of360° and it is at this centre of curvature 212 that the central axis 136of the guide tube 128 intersects the plane defined by the locating tool202.

The second arcuate member 216 is hinged with respect to the firstarcuate member 208 about an axis 220 that extends perpendicular to theplane defined by the locating tool 202 such that the second arcuatemember is moveable between an open position, shown in FIG. 17, and aclosed position shown in FIG. 18. In the open position an end 222 of thesecond arcuate member 216 remote from the hinge axis 220 is spacedsufficiently far from the handle 204 to permit the receipt of thefemoral neck. By contrast, in the closed position, the end 222 of thesecond arcuate member 216 abuts the handle 204 and the two arcuatesurfaces 210 and 218 define a closed circle. As with the head centeringjig of the first embodiment, in order to accommodate the range of femursizes present in the population, it will be necessary, in accordancewith the second embodiment, to provide a range of locating tools 202which differ in terms of the diameter of the circle defined by thearcuate surfaces 210 and 218 in this closed position. The length of thearm 126 would also need to vary from jig to jig in order to maintain therelationship between the central axis 136 of the guide tube 128 and itspoint of intersection with the plane defined by the locating tool 202.

In order to retain the second arcuate member 216 in the closed positionthe member is provided adjacent the end 222 with a bore 224 sized toreceive a head 226 of a locking pin 228. The body of the locking pin 228is substantially cylindrical and terminates at an end remote from thehead 226 in an axial projection 230. Approximately mid-way between thehead 226 and the axial projection 230 the locking pin 228 is providedwith a threaded bore 232 that extends perpendicularly to a longitudinalaxis of the locking pin.

The locking pin 228 is received within a stepped bore 234 provided inthe handle 204 which communicates with the bore 224 provided in thesecond arcuate member 216 when the second arcuate member is in theclosed position. Furthermore, the stepped bore 234 extends in adirection parallel to axis 203 and comprises a first region 236 having adiameter sufficient to receive the body of the locking pin 228 and asecond region 238 of reduced diameter remote from the bore 224sufficient only to receive the axial projection 230.

In order to assemble the locking pin 228, a helical spring or otherresilient member 240 is positioned over the axial projection 230 and thespring and locking pin 228 are inserted into the stepped bore 234 sothat an end of the axial projection protruding from the spring isreceived within the second region 238. In this way the spring is heldcaptive between an end of the body of the locking pin 242 and an annularshoulder 244 defined at the intersection of the first and second regions236 and 238 of the stepped bore 234. A slot 246 is provided in a sidewail of the handle 204 so as to communicate with the threaded bore 232.A threaded stud 248 may then pass through the slot 246 and into threadedengagement with the bore 232. Thus, whilst the locking pin 228 is biasedby the spring 240 in a first direction so as to cause the head 226 toproject out of the stepped bore 234 and be received within thecommunicating bore 224, movement of the stud 248 in the oppositedirection compresses the spring 240 and allows the head 226 to bewithdrawn from engagement with the second arcuate member 216. As will beapparent, the movement of the locking pin 228 is limited by theengagement of the stud 248 with the opposite ends of the slot 246.

If it is desired to be able to snap the second arcuate member 216 shutin the closed position without recourse to the stud 248 then this may beachieved by appropriate shaping of the head of the locking pin 226and/or by providing the end 222 of the second arcuate member with abeveled surface 250. In this way as the beveled surface 250 comes intocontact with the head of the locking pin 226 it pushes the locking pin228 into the handle 204 against the action of the spring 240 until suchtime as the bore 224 is brought into alignment with the stepped bore234.

In use, the head centering jig of the second embodiment is employed inthe same manner as that of the first embodiment to identify a point onthe femoral head in alignment with the central axis of the femoral neck.To this end the locating tool 202 is offered up to the femur with thesecond arcuate member 216 in the open position. Once the arcuate surface210 of the first arcuate member 208 has been placed up against thefemoral neck, the second arcuate member 216 is pivoted about hinge axis220 to the closed position to encircle the femoral neck. The secondarcuate member 216 is retained in the closed position with respect tothe first arcuate member 208 by the receipt of the head 226 of thelocking pin 228 within the bore 224.

Thereafter, as with the head centering jig of the first embodiment, theplane defined by the locating tool 202 is tilted by raising and loweringthe handle 204 until such time as the first alignment rod 114 is alignedparallel to the central axis of the femoral neck when viewed anteriorly.Assuming that the appropriately sized jig has been selected and that thefemoral neck is a close fit within the closed circle defined by arcuatesurfaces 210 and 218, the central axis 136 of the guide tube 128 willlie within a plane parallel to the coronal plane. By then aligning thefirst alignment rod 144, the central axis 136 is brought intocoincidence with the central axis of the femoral neck. If additionalalignment is required a second alignment rod 148 may be aligned with thecentral axis of the femoral neck when viewed laterally. Once the surgeonis satisfied that the central axis 136 of the guide tube 128 is alignedwith the central axis of the femoral neck he may use his free hand todrill the long pin through the guide tube 128 and into the femoral head.The head centering jig 200 is then disassembled and removed aspreviously described before performing the remaining steps of the HipResurfacing procedure.

A locating tool 302 according to a third embodiment of the presentinvention is shown in FIGS. 19 to 21. The locating tool 302 isconfigured to be used with a retaining member 114, guide tube 128 andtwo alignment rods 144 and 148 as described in relation to the firstembodiment and these features will not be described in detail inrelation to the third embodiment.

As shown in FIG. 19, like the first and second embodiments, the locatingtool 302 comprises an elongate handle 304. However, the locating tool302 of the third embodiment differs from that of the first and secondembodiments. In particular the locating tool 302 comprises a first arm340 and a second arm 370, as shown in FIGS. 20 and 21. The first andsecond arms 340 and 370 comprise respective first and second elongatehandles 342 and 372 which are designed to interlink to form the elongatehandle 304. Each of the first and second elongate handles 342 and 372are provided with apertures 115 and 119 for the receipt of theprojecting studs 118 and 120 provided on the under surface of theretaining member 114. The first and second elongate handles 342 and 372may also comprise the aperture 117 for use in securing the retainingmember 114 to the handle 304 by means of a locating screw 116 aspreviously described. Alternatively, a locking screw 201 similar to thatdescribed in relation to FIG. 18 may be used for the securement of theretaining member 114 to the handle 304.

In any event the apertures 115, 117 (if present) and 119 are designedsuch that when the first and second elongate handles 342 and 372 areinterlinked the apertures are mutually aligned.

The first and second arms 340 and 370 are hingedly attached by means ofa hinge 335 at a proximal end of the elongate handle 304 as shown inFIG. 21. However, it will be appreciated that such an attachment is notan essential feature of the invention. The hinge 335 preferablycomprises a pin 337 and a nut 338 as shown in FIG. 20.

As shown in FIGS. 20 and 21, the first elongate handle 342 comprises afirst surface 345 which merges at inner and outer edges 344 and 346 withinner and outer downwardly depending side walls 348 and 350,respectively. The outer side wall 350 in turn merges with a secondsurface 352 at a distance d1 from the first surface 344, which is shownas being parallel to said first surface 344, although it will beunderstood that such a parallel arrangement is not an essential featureof the invention. By contrast, the inner side wall 348 merges with aninwardly extending third surface 354 at a distance d2, d2 being lessthan the distance d1, to form a step 356. Again, although the thirdsurface 354 is depicted as being substantially parallel to the first andsecond surfaces 344 and 352 the invention is not so limited. The thirdsurface 354 terminates at an inner edge 358 at a lower inner downwardlydepending side wall 360 which in turn terminates with the second surface352.

In contrast to the first elongate handle 342, the second elongate handle370 comprises a first surface 372 which merges at inner and outer edges374 and 376 with inner and outer downwardly depending side walls 378 and380, respectively. The outer side wall 380 in turn merges with a secondsurface 382 at distance dl, from the first surface 372. By contrast, theinner side wall 378 merges with an outwardly extending third surface 384at distance d2 which in turn merges with a lower inner downwardlydepending wall 386 which merges with the second surface 382. The lowerinner wall 386 and third surface 384 form a step 388. As previouslydiscussed in relation to the first elongate handle 342, the first,second and third surfaces 372, 382 and 384 need not be in a parallelarrangement.

At a distal end, the first and second elongate handles 342 and 372 mergewith first and second arcuate members 308 a and 308 b which define firstand second arcuate surfaces 310 a and 310 b having a radius ofcurvaturer. When the locating tool 302 is in the closed position, asshown in FIG. 19, the first and second arcuate surfaces 310 a and 310 bhave a centre of curvature 312 coincident with the point at which thecentral axis 136 of the guide tube 128 intersects the plane defined bythe locating tool 302 and which lies on the longitudinal axis 103.

At their proximal ends, the first and second handle 342 and 372 aredesigned with tapers to allow for the first and second arms 340 and 370to rotate freely about the hinge 335.

The locating tool 302 is moveable between an open position, as shown inFIG. 21, and a closed position, as shown in FIG. 19. In the openposition the first and second arcuate members 308 a and 308 b aresufficiently spaced apart to permit the receipt of the femoral neck.Once the locating tool 302 is in position it is closed and held in theclosed position by the retaining member 114 which is mounted on theelongate handle 304. The locating tool 302 can be additionally held in aclosed position by the provision of a suitable closure means (not shown)at an end of the first and second arcuate members 308 a and 308 b remotefrom the first and second elongate handles 342 and 372. Alternativelythe locating tool 302 may be provided with arcuate members similar tothe arcuate members 208 and 216 described in relation to the secondembodiment.

As with the head centering jig of the first and second embodiments, inorder to accommodate the range of femur sizes present in the population,it will be necessary, in accordance with the third embodiment, toprovide a range of locating tools 302 which differ in terms of theradius of curvature r, and therefore the arcs subtended by the first andsecond arcuate member 308 a and 308 b in the closed position. The lengthof the arm 126 would also need to vary from jig to jig in order tomaintain the relationship between the central axis 136 of the guide tube128 and its point of intersection with the plane defined by the locatingtool 302.

Once the locating tool 302 is retained in the closed position by thefixation of the retaining member 114 to the elongate handle 304, as withthe head centering jig of the first or second embodiments, the planedefined by the locating tool 302 is tilted by raising and lowering thehandle 304 until such time as the first alignment rod 144 is alignedparallel to the central axis of the femoral neck when viewed anteriorly.Assuming that the appropriately sized jig has been selected and that thefemoral neck is a close fit within the substantially closed circledefined by arcuate surfaces 310 a and 310 b, the central axis 136 of theguide tube 128 will lie within a plane parallel to the coronal plane. Bythen aligning the first alignment rod 144, the central axis 136 isbrought into coincidence with the central axis of the femoral neck. Ifadditional alignment is required a second alignment rod 148 may bealigned with the central axis of the femoral neck when viewed laterally.Once the surgeon is satisfied that the central axis 136 of the guidetube 128 is aligned with the central axis of the femoral neck he may usehis free hand to drill the long pin through the guide tube 128 and intothe femoral head. The head centering jig 300 is then disassembled andremoved as previously described before performing the remaining steps ofthe Hip Resurfacing procedure.

Thus it will be apparent that the present invention as herein describedprovides a head centering jig that is less bulky than those of the priorart and which can be manipulated using only one hand. It will also beapparent that the use of the head centering jig no longer necessitatesthe exposure of the mid-lateral cortex of the femur.

1. A method of identifying a point on the femoral head in alignment witha central axis of the femoral neck, the method comprising the steps of:providing a jig comprising a first member having a locating member forat least partially receiving the femoral neck defining a plane, theplane being substantially perpendicular to the central axis of thefemoral neck, a guide member mounted in spaced relation with respect tothe first member and defining an axis at right-angles to the planedefined by the locating member, the axis intersecting the plane at alocation a predetermined distance from said locating member, and anelongate alignment member mounted with respect to the first member, theelongate alignment member being spaced from and extending parallel tothe axis defined by the guide member, exposing the femoral head andneck; offering up the first member to the femoral neck so that thefemoral neck is at least partially received by said locating member;aligning the elongate alignment member substantially parallel to thecentral axis of the femoral neck; and identifying an intersection pointat which the axis defined by the guide member intersects the femoralhead as the point on the femoral head in alignment with the central axisof the femoral neck.
 2. The method in accordance with claim 1, wherein amid-lateral cortex of the femur is not exposed.
 3. The method inaccordance with claim 1, wherein the first member is offered up to thefemoral neck and the elongate alignment member is aligned using only onehand.
 4. The method in accordance with claim 1, wherein the first memberis offered up to the femoral neck in a lateral approach.
 5. The methodin accordance with claim 1, wherein the step of offering up the firstmember to the femoral neck comprises substantially encircling thefemoral neck.
 6. The method in accordance with claim 1, wherein the stepof aligning the elongate alignment member comprises aligning theelongate alignment member with a central axis of the femoral neck whenviewed from two orthogonal directions.
 7. The method in accordance withclaim 1 and further comprising the step of drilling along the axisdefined by the guide member and into the femoral head.
 8. The method inaccordance with claim 1 and further comprising the step of disassemblingthe jig prior to removal from the femoral neck.
 9. The method inaccordance with claim 1, wherein the jig is provided as part of a kitcomprising a plurality of jigs having differing dimensions, the methodfurther comprising the step of selecting the appropriate jig for thesize of femoral head to be surveyed.
 10. A method of identifying a pointon the femoral head in alignment with a central axis of the femoral neckcomprising: providing a jig having a first member having a locatingmember disposed in a plane, the locating member comprising confrontingsurfaces adapted to substantially encircle the femoral neck, saidconfronting surfaces comprising first and second parts coupled by ahinge, at least one of said first and second parts being movable betweenan open position in which first and second parts are separated by adistance sufficient to receive the femoral neck and a closed position inwhich the first and second parts define a wall adapted to substantiallysurround the femoral neck; a guide member mounted in spaced relationwith respect to the first member and defining an axis at right-angles tothe plane defined by the locating member, the axis defined by the guidemember intersecting the plane at a location a predetermined distancefrom said locating member; and an elongate alignment member mounted withrespect to the first member, the alignment member being spaced from andextending parallel to the axis defined by the guide member; exposing thefemoral head and neck; offering up the first member having the first andsecond parts in an open position to the femoral neck so that the femoralneck is at least partially received by said locating member; aligningthe elongate alignment member substantially parallel to the central axisof the femoral neck; and identifying an intersection point at which theaxis defined by the guide member intersects the femoral head as thepoint on the femoral head in alignment with the central axis of thefemoral neck.
 11. The method in accordance with claim 10, wherein theguide member comprises a hollow tube and the axis defined by the guidemember comprises a central axis of the tube, and the identifying stepincludes identifying the intersection point through the hollow tube. 12.The method in accordance with claim 10 further including the step ofselectively detaching the guide member from the first member.
 13. Themethod in accordance with claim 10, wherein the step of aligning theelongate alignment member comprises aligning the elongate alignmentmember with a central axis of the femoral neck when viewed from twoorthogonal directions.
 14. The method in accordance with claim 10further comprising the step of drilling along the axis defined by theguide member and into the femoral head.
 15. The method in accordancewith claim 10 further comprising the step of disassembling the jig priorto removal from the femoral neck.
 16. A method for identifying a pointon a femoral head in alignment with a central axis of an associatedfemoral neck comprising: a jig including a first member having alocating member disposed in a plane to at least partially receive thefemoral neck; a guide member mounted in spaced relation with respect tothe first member and defining an axis at right-angles to the planedefined by the locating member, the axis defined by the guide memberintersecting the plane at a location a predetermined distance from saidlocating member, said axis defined by the guide member beingsubstantially coincident with the central axis of the received femoralneck; and an elongate alignment member mounted with respect to the firstmember, the alignment member being spaced from and extending parallel tothe axis defined by the guide member, said elongate alignment membercomprising first and second rods mounted with respect to the firstmember and extending parallel to and spaced from the axis defined by theguide member, the first rod being spaced from said axis in a directionorthogonal to that in which the second rod is spaced from said axis;exposing the femoral head and neck; offering up the first member havingthe first and second parts in an open position to the femoral neck sothat the femoral neck is at least partially received by said locatingmember; aligning the elongate alignment member substantially parallel tothe central axis of the femoral neck; and identifying an intersectionpoint at which the axis defined by the guide member intersects thefemoral head as the point on the femoral head in alignment with thecentral axis of the femoral neck.
 17. The method in accordance withclaim 16, wherein the guide member comprises a hollow tube and the axisdefined by the guide member comprises a central axis of the tube, andthe identifying step includes identifying the intersection point throughthe hollow tube.
 18. The method in accordance with claim 16, wherein thestep of aligning the elongate alignment member comprises aligning theelongate alignment member with a central axis of the femoral neck whenviewed from two orthogonal directions.
 19. The method in accordance withclaim 16 further comprising the step of drilling along the axis definedby the guide member and into the femoral head.
 20. The method inaccordance with claim 16 further comprising the step of disassemblingthe jig prior to removal from the femoral neck.